Show user popover for sidebar participants list

What does this MR do and why?

Note: This MR is a part of a series of MRs where we're making popover behaviour consistent across GitLab.

This MR adds support for showing user popover for sidebar participants list when hovered over a participant avatar in the sidebar instead of plain tooltip.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

	Before	After
Issue/MR Sidebar
Design Discussions

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Visit any Issue, MR, or an Epic and hover over the avatars within participants list on right sidebar to reveal user popover.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related Issue

#416174 (closed)